This invention relates to a solid-state laser device, more particularly, to a solid-state laser device producing, by using a semiconductor laser device as an excitation light source, an output laser beam.
As well known in the art, a solid-state laser device of the type described comprises a solid-state laser medium disposed in a laser resonator and a semiconductor laser device for exciting the solid-state laser medium by applying an excitation laser beam to the solid-state laser medium to make the solid-state laser medium produce an output laser beam. The excitation laser beam has an excitation wavelength while the solid-state laser medium has an absorption wavelength or an absorption band. If the excitation wavelength does not equal to the absorption wavelength, the solid-state laser device produces the output laser beam with an oscillation efficiency thereof remarkably degraded. It is noted that the excitation wavelength of the excitation laser beam varies in accordance with temperature of the semiconductor laser device. It is therefore necessary to accurately keep the semiconductor laser device at a predetermined desired temperature so that the excitation wavelength of the excitation laser beam equals the absorption wavelength of the solid-state laser medium.
A method of keeping the semiconductor laser device at the predetermined desired temperature is already proposed in U.S. Pat. No. 4,739,507 issued to Byer et al. According to Byer et al, the semiconductor laser device such as a laser diode is cooled by means of a thermoelectric cooler such as a Peltier effect element coupled in good heating exchanging relation with the laser diode via the intermediary of a substrate such as a copper block. In order to obtain the output laser beam at high power, it is necessary for the solid-state laser device to use a number of semiconductor laser devices. Under the circumstances, the semiconductor laser devices must be provided with a number of Peltier effect elements and a number of driving units for driving the Peltier effect elements, respectively. Such a solid-state laser device becomes complex in structure and has a high price. It is therefore difficult for the solid-state laser device to put to practical use.
Another example is disclosed in the technical digest Advanced Solid-State Laser 1991, pdp8-1. According to this technical digest, a radiator is mounted on a substrate on which a plurality of semiconductor laser devices are fixed and cooling water passes through the radiator.
However, each of the above-mentioned conventional methods of cooling the solid-state laser device comprises the steps of adjusting temperature of the substrate so as to maintain the desired temperature and carrying out heat conduction between contact surfaces of the substrate and of the semiconductor laser device or the semiconductor laser devices, whereby temperature of the semiconductor laser device is indirectly controlled so as to maintain the desired temperature. Accordingly, each of the conventional cooling methods is called an indirect cooling method. In such an indirect cooling method, there is a thermal or heat resistance between the contact surfaces. As a result, the indirect cooling method has a poor efficiency and a low stability for control of the temperature. By way of example, it will be assumed that the semiconductor laser device having power of 10 W is drived at the temperature of 20.degree. C. In this event, the semiconductor laser device generates heat of about 30 W. It will also be assumed that the contact surfaces between the substrate and of the semiconductor laser device has the heat resistance of 0.2.degree. C./W. Under the circumstances, it is necessary for the temperature of the substrate to degrade to 14.degree. C. The larger the power of the semiconductor laser device becomes, the wider a temperature difference between the semiconductor laser device and the substrate becomes. Accordingly, a problem occurs in a case where the solid-state laser device comprises the semiconductor laser device having a high power.
As mentioned above, in the conventional cooling methods, it is indispensable to take radiation of heat for the semiconductor laser device into consideration. Therefore, there is a lot of restriction concerned with problems of heat in structure of the solid-state laser device. In addition, limitations lie in as regards the power of the semiconductor laser device to be used and the number of the semiconductor laser devices to be arranged. Furthermore, the solid-state laser device is complex in structure.
In addition, in the conventional solid-state laser device, the solid-state laser medium is merely supported with a supporting member without an active cooling. With this structure, the solid-state laser medium is only cooled at a portion which is in contact with the supporting member. As a result, an irregular temperature distribution easily causes inside the solid-state laser medium to result in distortion due to heat for the solid-state laser medium. Accordingly, such distortion readily gives rise to deterioration in the quality of the output laser beam and to degradation of the solid-state laser medium.